Existing techniques for tracking the sun rely typically on one or more of three methods. The diurnal motion of the sun is well understood, and consequently a telescope, for example, can be mounted on an accurately aligned alt-azimuth or equatorial mount. The axial drives of that mount are then computer controlled to maintain the telescope in an orientation that will point the objective lens or mirror of the telescope at the sun's calculated position.
This approach, however, requires the highly accurate initial alignment of the mount. This may be practical in a fixed instrument such as research telescope, where the accurate alignment of the mount is one facet of an overall extensive ad precise installation procedure conducted by expert scientists and engineers, of lengthy duration and considerable expense. Such installation time and expense may not be acceptable in other applications, such as the installation of solar power collectors on a mass scale.
A less accurate and cheaper alternative is to control the axial drives on the assumption that the sun follows the ecliptic (or even celestial equator) in an entirely regular manner, thug ignoring the effects of the equation of time and—where the sun is assumed to follow the celestial equator—the effects of the earth's axial tilt.
Another existing approach is so-called shadow bar sun sensing, in which a pair of sensors are mounted on a solar radiation collector (such as a dish or plane mirror) between a shadow bar. The shadow bar casts a shadow on one of the sensors if the collector is not pointing directly at the sun. The collector's attitude can then be adjusted on the basis of the outputs of these sensors until those outputs are equal.
These existing approaches, however, make no allowance for the subsequent effects of imperfect manufacturing tolerances on the orientation of the radiation receiver (to which the collector directs collected radiation) relative to the collector itself. The effect of such imperfections will also vary with the changing position of the sun and orientation of the collector, even if the receiver is fixed with respect to the collector. In fact the receiver may also shift slightly relative to the collector, owing to sagging in the receiver supports (which would commonly be used to hold the receiver at the focus of the collector), or to variations in the overall structure due to temperature fluctuations and the like.
For many applications these shortcomings may be acceptable, or at least tolerable, especially in systems where maximizing the collection of solar radiation is less sensitive to tracking precision. This may be the case in systems that do not concentrate the solar flux by means of, for example, a spherical or parabolic mirror. If a plane mirror is used, errors in tracking precision of even 5° may not excessively reduce collection efficiency. Indeed, many solar hot water heaters (typically with flat collection panels) perform no solar tracking whatsoever, so existing approaches—which provide at least some tracking—will clearly be of use in some applications. However, where the solar flux is concentrated (possibly by a factor of as much as three or more), a 5° tracking error may produce unacceptably high looses in collection efficiency.